The invention relates to an electric lamp comprising
a glass lamp vessel which is closed in a gastight manner and in which an electric element is accommodated,
current conductors connected to the electric element which each have an end portion of molybdenum projecting to outside the lamp vessel, said end portion being provided with means for protection against oxidation.
Such an electric lamp is known from EP 573 114.
Current conductors with molybdenum end portions are often used in electric lamps because this metal is resistant to high temperatures and because this metal has a coefficient of expansion which matches that of hard glasses well and deviates only little from that of quartz glass, i.e. glass with an SiO2 content of at least 95% by weight.
It is a disadvantage of molybdenum, however, that it readily oxidates at room temperature already, so that there is a considerable risk that a good electrical contact with, for example, the connection terminals of a lampholder will be lost.
According to the cited patent document, the end portions are provided with a molybdenum nitride coating. A disadvantage of the known lamp is, however, that an oxidation resistance up to no more than a comparatively low temperature, i.e. approximately 200xc2x0 C., is obtained. Furthermore, the coating has the additional disadvantage that the end portions become more liable to fracture.
It is an object of the invention to provide an electric lamp of the kind described in the opening paragraph in which the above disadvantages are counteracted.
According to the invention, this object is achieved in that the electric lamp of the kind described in the opening paragraph is characterized in that the end portion has a skin which is chosen from a group of materials formed by titanium nitride and chromium carbide.
The titanium nitride or chromium carbide skin is not only easy to realize, but it is also an effective agent against oxidation not only at room temperature but also at elevated temperatures, for example up to approximately 400xc2x0 C. Titanium nitride and chromium carbide, moreover, have the advantages that they do not lead to an increased brittleness of the molybdenum end portion and that they are thermally stable also at very high temperatures, for example 2000xc2x0 C. That is to say that titanium nitride and chromium carbide substantially do not form compounds or alloys with molybdenum which melt at lower temperatures than those used in the manufacture of the lamp. The thermal stability at very high temperatures also implies that no dissociation of the compounds occurs owing to the high temperature, leading to compounds which are unsuitable for the oxidation-resistant coating. This renders said compounds suitable for use as a skin on metal parts which is effective against oxidation, for example in lamps, for example quartz glass lamps, for which very high temperatures are used in the lamp manufacturing process.
Preferably, the skin has a layer thickness of at least 2 xcexcm and at most 3 xcexcm. A layer thickness below 2 xcexcm provides the molybdenum with an insufficient protection against oxidation. A layer thickness above 3 xcexcm is unnecessarily expensive because it does not provide any better protection against oxidation than a skin with a layer thickness of 3 xcexcm.
The oxidation-resistant skin on the molybdenum end portion may be readily obtained in a vapor deposition process, for example a CVD process. The CVD process has the advantage that many molybdenum end portions can be vaporized simultaneously in one and the same process. A molybdenum end portion provided with an oxidation-resistant skin can thus be manufactured comparatively inexpensively.
In spite of the protection against oxidation provided by the titanium nitride or chromium carbide skin, the protected end portion can be processed in a conventional manner, for example by welding to a metal foil, for example to a molybdenum foil on which a gastight seal of the lamp vessel is realized. A good electrical connection, which is only a few mxcexa9 larger than in the case of platinum or platinum-plated end portions, can be realized on the protected end portion, for example by means of contacts of a lampholder.
The electric element of the lamp may be a pair of electrodes in an ionizable gas or an incandescent body, for example in an inert gas comprising halogen. The lamp vessel may have one or several seals from which a current conductor issues to the exterior. The lamp vessel, for example made of quartz glass or hard glass, may be joined together with a reflector body so as to form a lamp.